1. Field of the Invention
The present invention relates to a lubrication structure in an engine, and more specifically, to a lubrication structure of a connecting rod big-end in a splash lubrication engine.
2. Discussion of the Related Art
In an engine, a piston contained in a cylinder is connected to a crank shaft via a connecting rod. The connecting rod has a small-end connected to the piston so as to allow the piston to make a reciprocating movement in a bore of the cylinder part. The connecting rod has a big-end connected to a crank pin in a crank case, so as to rotate with respect to each other.
FIG. 8 is a diagram for explaining a known method for lubricating a connecting rod big-end and a crank pin, as described in Japanese Kokai Publication 1996-284944. In FIG. 8, a ring-shaped big-end 102 of a connecting rod 101 is divided into a big-end first half 103 and a big-end second half 104 along a diameter line of an inner peripheral surface of the big-end 102. A crank pin 106 of a crank shaft 105 is received by the big-end first and second halves 103 and 104 and connected thereto by a bolt. The big-end 102 is put between a pair of crank webs 107, which are opposes to each other. Surfaces of the big-end first half 103, formed in the axial direction, face the thrust receiving surfaces 107a of the crank webs 107 to contact therewith. On the other hand, the big-end second half 104 is structured to have a width in an axial direction which is narrower than that of the big-end first half 103. Spaces as oil grooves 108 are formed between surfaces of the second half 104, and the thrust receiving surfaces 107. The oil grooves 108 formed on both sides of the second half 14 are in the form of half rings. Namely, each of the oil grooves 108 is defined by a surface of the big-end second halve 104, the trust receiving surface 107 and the outer surface of the crank pin 106.
The thus formed oil grooves 108 in the form of half rings receive oil splash caused by the rotation of a crank shaft 105. The oil in the oil grooves 108 lubricates the outer surface of the crank pin 106, the inner peripheral surface of the connecting rod big-end 102, and the thrust receiving surfaces 107a of the crank webs 107.
As schematically shows in FIG. 9, another known structure shows that a big-end 112 of a connecting rod 101 is divided into a big-end first half 113 and a big-end second half 114 along a diameter line of an inner peripheral surface 112a of the big-end 112. A crank pin 122 of a crank shaft 121 is received in the big-end first and second halves 103 and 104 and connected thereto by a bolt. A first oil groove 115 is formed on the inner peripheral surface 112a of the big-end 112. A second oil groove 116 is perforated in the big-end 112 which communicates the outside of the big-end 112, i.e., inner space in a crank case with the first oil groove 115.
According to the structure shown in FIG. 8, however, it is possible that sufficient lubrication is not provided. This is because the spaces defined by the outer surfaces of the crank pin 106 and the inner peripheral surface of the big-end 102 is extremely small. Therefore, it is difficult to provide a space with a size which is sufficient to permit a proper amount of oil to the center, in an axial direction, of the big-end.
In the lubrication structure shown in FIG. 9, the oil stored in the oil groove 115 is pushed to the side of the bottom of the oil groove 115 because of centrifugal force. This makes it difficult to supply a sufficient amount of oil to a space between the outer surface 122a of the crank pin 122 and the inner peripheral surface 112a of the big-end 112.
Moreover, the surface areas as sliding surfaces between the outer surface 122a of the crank pin 122 and the inner peripheral surface 112a of the big-end 112 is decreased by the area of the oil groove 115. In this case, the pressure applied to the sliding surfaces is increased, and hence the relative rotation tends to be less smooth. In this way, satisfactory lubrication may not be achieved.